1. Field of the Invention
The present invention relates to a system for transferring and sorting workpieces, and in particular to a transfer and sorting system including a robot having a dual paddle end effector capable of affecting parallel processing of workpieces.
2. Description of Related Art
The introduction of workpiece handling robots into the semiconductor fabrication process represented a significant advance in automation over manual and early transfer equipment for moving workpieces such as semiconductor wafers between various processing tools and/or workpiece storage locations. It is an important feature of conventional workpiece handling robots to be able to quickly and precisely acquire a workpiece from a first position, deliver it to a new location having different X, Y and Z coordinates in Cartesian space, and set it down without risk of damage to the workpiece.
In order to accomplish this, a typical robot includes a central mast mounted in a base for translation along a vertical axis. A proximal arm, or link, is rotatably mounted to an upper end of the mast, and a distal arm, or link, is rotationally mounted to the opposite end of the proximal link. The wafer handling robot further includes an end effector attached to the distal link for supporting the workpiece. The end effector is pivotally mounted to the distal link for synchronized motionwith the distal and proximal links. Various motors are further provided, conventionally mounted in the base, for translating the central mast, and for rotating the proximal and distal links such that the end effector may be controllably maneuvered in three-dimensional space.
In addition to their use in processing tools for forming the integrated circuits on the wafers, workpiece handling robots are used in stand alone processing tools such as for example wafer sorters. Wafer sorters are used for a variety of purposes in a fab, such as for example to transfer one or more wafers between various cassettes positioned on the wafer sorter. During this process, wafers from a number of cassettes can be combined into one cassette, and wafers from one cassette can alternatively be split up among a number of cassettes. The wafers can also be transferred between the cassettes in the same order or reordered as desired. Another function of a wafer sorter is to map the location of wafers within a cassette, and to detect incorrect positioning of wafers within a cassette.
A wafer sorter may further include an aligner. Conventional aligners have a chuck for supporting and rotating a wafer and typically two cameras, one for identifying a radial runout (i.e., a magnitude and direction by which the workpiece deviates from an expected centered position on the chuck), and for identifying the position of a notch located along the circumference of the wafer. An aligner typically includes a second camera for reading an optical character recognition (OCR) mark which identifies the workpiece. In a conventional wafer sorter, wafers are transferred one at a time to the chuck of the aligner which then rotates the wafer to determine the radial runout, identify the location of the notch, and to read the OCR mark. The wafers are then returned one at a time to the wafer cassette. Conventional aligner/robot systems which transfer workpieces one at a time between the workpiece cassette and aligner have a relatively low throughput, on the order of approximately 200-250 wafers per hour. This low throughput is significant as the alignment process must be performed for each processing station where an indicial mark reading is required, and must be performed on each individual wafer at each of these stations.
It is known to provide robots having two separate sets of robotic arms, or links, to increase throughput. One such multiple armed robot is disclosed in U.S. Pat. No. 5,789,890 to Genov et al. As disclosed therein, such robots typically include end effectors offset from each other so as to be able to obtain a first workpiece from the cassette, spin around, and then acquire a second workpiece. Such robots take up a significant amount of space within the sorter, where space is at a premium owing to the expense of maintaining sorters with ultraclean air. Moreover, typical dual armed robots are expensive, and require complicated controls.